1. The Field of the Invention
The invention relates to a control method for a cyclone separator apparatus attached directly to a fluid catalytic cracking (FCC) riser reactor.
2. The Related Art
U.S. Pat. No. 5,248,411 describes an apparatus for rapidly separating catalyst from a cracked hydrocarbon gas in a fluidized catalytic cracking (FCC) unit. It also describes a process for withdrawing stripper gas from an FCC reactor vessel. A vent orifice for withdrawal of reactor and stripper gasses is located in an annular space formed around the riser cyclone outlet tube and the roof of the riser cyclone. The vent orifice provides pressure stability in a direct-coupled cyclone system.
The fluid catalytic cracking (FCC) process comprises mixing hot regenerated catalyst with a hydrocarbon feedstock in a transfer line riser reactor under catalytic cracking reaction conditions. The feedstock is cracked to yield gasoline boiling range hydrocarbon as well as degradation products, such as coke, which deposits on the catalyst causing a reduction in catalytic activity. Hydrocarbon vapor and coked catalyst are passed from the top of the riser reactor to a containment vessel, containing a cyclone separator, wherein catalyst is separated from hydrocarbon. In the art, the separator vessel is termed the reactor vessel or the disengager vessel. The separated catalyst is passed to a stripper, also in the containment vessel, and contacted with a stripping gas to remove volatile hydrocarbon. Stripped catalyst is then passed to a separate regeneration vessel wherein coke is removed from the catalyst by oxidation at a controlled rate. Catalyst, substantially freed of coke, is collected in a vertically oriented regenerated catalyst standpipe. The regenerated catalyst is passed from the standpipe to the riser reactor for cyclic reuse in the process.
A conventional fluid catalytic cracking (FCC) feedstock comprises any of the hydrocarbon fractions known to yield a liquid fuel boiling range fraction. These feedstocks include light and heavy gas oils, diesel, atmospheric residuum, vacuum residuum, naphtha such as low grade naphtha, coker gasoline, visbreaker gasoline and like fractions from steam cracking.
Catalyst development has improved the fluid catalytic cracking (FCC) process. High activity, selectivity and feedstock sensitivity are demonstrated by the new crystalline zeolite cracking catalysts. These high activity catalysts have been used to improve the yield of more desirable products.
The hydrocarbon conversion catalyst employed in a fluid catalytic cracking (FCC) process is preferably a high activity crystalline zeolite catalyst of a fluidizable particle size. The catalyst is transferred in suspension or dispersion with a hydrocarbon feedstock, upwardly through one or more riser conversion zones which provide a hydrocarbon residence time in each conversion zone in the range of 0.5 to about 10 seconds, typically less than about 8 seconds. High temperature riser hydrocarbon conversion occurs at temperatures of at least 900.degree. F. (482.degree. C.) up to about 1450.degree. F. (788.degree. C.), pressures of 5 psig (1.3 atm) to 45 psig (4 atm) and at 0.5 to 4 seconds hydrocarbon residence time with catalyst in the riser. The vaporous hydrocarbon conversion product is rapidly separated from catalyst.
In modern fluid catalytic cracking (FCC) units, cracking temperature has been increased to obtain high conversion of feedstock boiling range material to light products. Typical cracking temperatures in modern fluid catalytic cracking (FCC) units are in the range of 980.degree. F. (526.degree. C.) to 1050.degree. F. (565.degree. C.), or above. At these high temperatures, thermal degradation of cracked liquid products can be significant, resulting in formation of additional gaseous products and loss of valuable liquid products. In many cases the fluid catalytic cracking (FCC) unit capacity and operating severity are limited by the ability to compress and recover the light gaseous products.
Rapid separation of catalyst from hydrocarbon product is particularly desirable to limit hydrocarbon conversion time to the residence time in the riser conversion zone. During the hydrocarbon conversion, coke accumulates on the catalyst particles and entrains hydrocarbon vapors. Entrained hydrocarbon contact with the catalyst continues after removal from the hydrocarbon conversion zone until the hydrocarbon is separated from the catalyst. Allowing the catalytic reaction to proceed beyond the optimum contact time results in degradation of liquid products to less desirable gaseous products and coke.
Catalyst is separated from hydrocarbon by cyclone separating and then stripped with stripping gas to remove volatilizable hydrocarbon. Hydrocarbon conversion products and stripped hydrocarbon are combined and passed to a fractionation and vapor recovery system. This system comprises a fractionation tower, vapor coolers and wet gas compressor operated at a suction pressure of 0.5 psig (1.03 atm) to 10 psig (1.7 atm). Stripped catalyst containing deactivating amounts of coke, is passed to a catalyst regeneration zone.
One or more cyclone separators are used to provide a rapid, efficient separation of cracked hydrocarbon from catalyst particles at the outlet of the riser reactor. These cyclone separators, usually designated as riser our rough-cut cyclones, terminate the catalytic reactions taking place in the riser reactor. Riser cyclones may be either external, or more commonly, internal to the reactor vessel. The separated vapor from riser cyclones is typically discharged into the upper section of the reactor vessel and passed to one or more sets of secondary cyclones for removal of catalyst particles before the vapors enter the fractionation and vapor recovery system. In fluid catalytic cracking (FCC) units operating at cracking temperatures above about 980.degree. F. (526.degree. C.) significant thermal degradation of cracked products can occur when the vapors are allowed to enter the reactor vessel. To reduce thermal degradation of cracked products, direct-coupled or closed cyclone systems, such as disclosed in U.S. Pat. No. 5,248,411 have been used. In direct-coupled cyclones, the separated vapors from the riser cyclones are passed directly to the inlet of secondary cyclones. Direct-coupled cyclones reduce thermal degradation of cracked products by shortening the residence time of the vapor.
An object of the present invention is to provide a direct-coupled cyclone apparatus for rapidly separating the catalyst-hydrocarbon suspension and preventing thermal degradation of cracked products. Another object of this invention is to provide a direct-coupled cyclone system for use with riser cyclones external to the reactor or disengager vessel. And yet another object of this invention is to provide a control method to establish and maintain a stable pressure gradient between the riser cyclone barrel and the reactor vessel to facilitate removing stripper gas from the reactor vessel.
U.S. Pat. Nos. 4,623,446 and 4,737,346 to J. H. Haddad et al. teach a closed-coupled cyclone separator system in the reactor vessel of a fluid catalytic cracking apparatus. Means is provided for blending stripping gas with cracked hydrocarbon as it flows to a directly coupled riser cyclone separator.
U.S. Pat. No. 4,502,947 to Haddad et al. discloses a closed cyclone fluid catalytic cracking catalyst separation method and apparatus. In the closed cyclone, hydrocarbon product and catalyst are passed directly into a cyclone separator from a riser without passing into the atmosphere of the reactor vessel. Avoiding the atmosphere of the reactor vessel reduces both excess catalytic cracking and high temperature thermal cracking.
U.S. Pat. No. 5,221,301 to N. L. Giuricich discloses a multistage cyclone separator system with a plenum providing a manifold and structural support.
There is a need in the art to capture the process advantages of U.S. Pat. No. 5,248,411 in existing fluid catalytic cracking (FCC) reactor vessels where limited free volume restricts retrofit of a direct-coupled riser cyclone.